Steam derived from a fossil-fueled boiler is generally hot and dry and contains sufficient energy to operate the high-pressure turbine. Thereafter it is generally reheated in the boiler so that sufficient useful work may be performed thereby, first in intermediate and then in low-pressure stages. Steam from a nuclear steam generator or reactor, on the other hand, is generally of relatively low temperature and is saturated. After passing through a high-pressure turbine stage the nuclear steam contains sufficient entrained moisture that it must be demoisturized, and preferably reheated thereby increasing its enthalpy in order that it reliably perform further useful work.
A separate heat exchanger, a Moisture Separator Reheater (MSR), is used for this purpose. In an MSR, moisture entrained in the once-used, or shellside, steam is first removed by passage of the steam through panels of a moisture separator containing a series of channels between angular plates with large surface area for removing entrained moisture. The then-dried steam containing about 0.5% moisture is passed in heat-exchange relationship with one or more tube bundles generally of re-entrant or U-shaped configuration, which contain steam taken from the turbine inlet or from an extraction port in the high-pressure turbine (tubeside steam), are reheated to a temperature at which it may pass to the low-pressure turbine to reliably do useful work.
The design objective of the reheater tube bundles is to raise the temperature of the shellside steam to as near a value of the tubeside steam as possible. As a practical matter, sufficient heat transfer surface is provided so that it is raised to within approximately 25.degree. F. of that value.
Ideally one would seek to supply sufficient steam to each tube in the tube bundles so that, after heat exchange with the shell-side steam via condensation, the tubeside steam is completely condensed at the tube exit. In practice, some tubes are supplied with excess steam and others are supplied with insufficient steam which leads to condensate subcooling and possible instabilities.
Accordingly, it is an object of the invention to provide a moisture separator reheater construction which substantially eliminates the existence of subcooled condensate in the reheater tubes thereof.
Another object of the invention is to provide improved moisture separator reheater structure which includes improved scavenging of the reheater tube bundles thereof.
Yet another object of the invention is to provide moisture separator reheater structure, and method of operating thereof, which substantially eliminates subcooling of condensate in the tube bundles thereof with little or no loss in thermodynamic efficiency thereof.